Figuring out MISFIRES

    Misfire is a very common driveability problem that might be simple to identify, with respect to the cause. A misfiring cylinder inside a four-cylinder engine is, pardon the pun, difficult to miss. Losing 25% from the engine's energy output is something like a equine attempting to operate on three legs. The engine may shake badly at idle it causes oscillations that may be felt within the controls and through the vehicle. The engine also might be difficult to start and might stall at idle, with respect to the accessory load (ac, car headlights and electric rear defroster, for instance).

When misfire happens, performance suffers together with gas mileage, pollutants and idle quality. And, whenever a misfiring vehicle is exposed for an pollutants test, it'll usually fail due to the abnormally high amounts of hydrocarbons (HC) within the exhaust.

What can cause a cylinder to misfire? Essentially, it's 1 of 3 things: lack of spark the environmentOrgas mixture is simply too far out of whack to ignite or lack of compression. Lack of spark includes something that prevents coil current from jumping the electrode gap in the finish from the spark plug. Causes include worn, fouled or broken spark plugs, bad plug wires or perhaps a cracked distributor cap. An inadequate coil or excessive rotor gas in the distributor would affect all cylinders, not only a single cylinder.

"Lean misfire" can happen once the air/fuel mixture is simply too lean (insufficient gasoline within the mixture) to lose. This is often triggered with a dirty, clogged or inoperative fuel injector air leaks or low fuel pressure due to an inadequate pump, restricted filter or leaking pressure regulator. Low fuel pressure would affect all cylinders instead of a person cylinder, as would most air leaks. A leaking EGR valve may also have a similar effect being an air leak.

Lack of compression means the cylinder manages to lose the majority of its air/fuel mixture prior to it being captivated. Probably the most likely causes listed here are a leaking (burned) exhaust valve or perhaps a blown mind gasket. If two adjacent cylinders are misfiring, it's likely the mind gasket together has unsuccessful. Also, if the engine is getting too hot or losing coolant, it's likely the mind gasket may be the reason.

Intermittent misfires would be the worst kind to identify since the misfire is inconsistent based on engine load or operating conditions. They appear to happen without no reason. The engine may misfire and run rough when cold however lessen because it gets warm. Or, it might start and idle fine however misfire or hesitate as it pertains under load. Also, it might run fine more often than not but all of a sudden misfire or eliminate without no reason. Intermittent misfires could be a real challenge to identify, so let's begin with a stable misfire in a single cylinder before moving onto intermittent misfires.

STEADY MISFIRE

Within the situation of the steady misfire, separating the misfiring cylinder is the initial step in figuring out the issue. Today's OBDII systems get this to easy utilizing a scantool for example AutoTap. Simply employ AutoTap to see the Diagnostic Trouble Code (DTC) saved within the PCM.

This really is more suitable towards the traditional approach to tugging plug wires to recognize the weak cylinder, since it prevents the current from leading to any harm to the electronics within the ignition system. Whenever a plug wire is physically disconnected from the spark plug, our prime current surge in the coil cannot follow its normal road to ground with the plug wire and spark plug, therefore it passes back with the coil. Most ignition systems are robust enough to resist such current backup copies occasionally but this is not on an extended basis. When the coil or ignition module has already been weak, it might push the component within the edge leading to it to fail.

IGNITION/COMPRESSION Inspections

So, now you've identified a misfire and also have isolated it to 1 cylinder. Many occasions, the reason is going to be apparent whenever you take away the spark plug. When the plug's insulator is cracked or damaged, you've found the issue. When the plug seems to become OK but is wet, inspect the plug wire and boots for damage. Appraise the plug wire's resistance, finish to finish, by having an ohmmeter. Make reference to the automobile manufacturer's specifications, but, usually, resistance shouldn't exceed 8,000 ohms per feet. Switch the wire if resistance surpasses specifications. When the plug is fouled, you've found the origin from the misfire, but you've still got to determine which triggered the plug to foul. Heavy black oily carbon deposits would tell you just how the engine is burning oil. Probably the most likely cause is worn valve guide closes and/or guides, but worn rings and cylinders may also allow oil to go in the combustion chamber. Changing the spark plug will temporarily cure the misfire problem, but, before the oil consumption issue is fixed, the engine continues to foul plugs.

A leakdown or compression test can help you determine whether the oil gets beyond the valve guides or even the rings. When the cylinder shows little leakdown or holds good compression whenever a little oil is squirted in to the cylinder (wet compression test), it might tell you just how the engine needs new valve guide closes and/or guide work. Most late model import engines have positive valve guide closes. Frequently, the guides are fine, however the closes are worn or cracked. The closes could be changed on some engines without an excessive amount of effort and without needing to take away the mind.

Just accomplish the valve cover, take away the valvetrain hardware and employ an exterior spring compressor to get rid of the springs so new closes could be installed. A controlled air hose attached to the spark plug hole could keep the valve from shedding in to the cylinder. But, on many OHC engines, there's a lot disassembly involved to get at the valve springs you're best getting rid of the mind.

A spark plug that shows heavy whitish to brown deposits may suggest a coolant leak either beyond the mind gasket or via a crack within the combustion chamber. This kind of problem is only going to worsen and could soon result in increased problems when the leak isn't fixed. Coolant constitutes a lousy lubricant and may cause ring, cylinder and bearing damage whether it will get right into a cylinder or even the crankcase. Lack of coolant may also result in getting too hot, which may lead to cracking or bending of aluminum cylinder heads. If you think this type of problem, pressure test the air conditioning to check on for internal coolant leakage. Spark plugs that demonstrate preignition or detonation damage may suggest a necessity to check on timing, the whole process of the air conditioning and types of conditions that create a lean air/fuel mixture. You could also wish to change to a cooler warmth range plug.

Short trip stop-and-go driving may cause an immediate buildup of normal deposits on plugs, particularly if the engine provides extensive miles and there's been some oil leakage beyond the valve guide closes and rings. Solution here may be to change to some one-step warmer spark plug.

When the spark plug and plug wire are OK however the cylinder is weak, a leakdown or compression test ought to be done to find out if the issue is compression related. The exhaust valves are the type probably to get rid of their seal and leak compression, so, if you discover abnormally low compression, follow-up having a wet compression test to find out when the problem lies using the valves or rings. No alternation in compression having a wet test would let you know the issue is valve related (most likely a poor exhaust valve) or perhaps a blown mind gasket. But, when the compression blood pressure measurements are considerably greater having a wet compression test, it might let you know the piston rings and/or cylinder walls are worn. In either case, you're searching at major repairs. The only real remedy for a leaking valve is really a valve job, and also the only remedy for a leaking mind gasket would be to switch the gasket. Likewise, the only real remedy for worn rings and cylinders would be to overhaul or switch the engine.

Low compression may also be triggered with a rounded cam lobe. When the valve doesn?¡¥t open, the cylinder can't breathe normally and compression is going to be low. A visible inspection from the valvetrain and cam is going to be necessary if you think this type of problem.

INJECTOR Inspections

When the ignition components and compression inside a misfiring cylinder are fine, that leaves fuel (or even the lack thereof) because the only other possibility. You can begin by checking for current in the injector. A great injector also needs to buzz as the engine is running. No buzzing would let you know the injector is dead, while a no-current reading through would let you know it isn't the injector's fault but a wiring or computer driver problem.

When the injector is buzzing and squirting fuel however the cylinder isn't getting enough fuel, the injector is dirty or clogged. On-vehicle cleaning might help take away the varnish deposits which are restricting the injector and restricting fuel delivery.

If you're handling a random misfire that can't be isolated to particular cylinder, all of the injectors might be dirty. It's also wise to check fuel pressure to ascertain if the pump is weak or even the pressure regulator is defective. A blocked fuel filter can help to eliminate fuel pressure. If fuel pressure is at specifications, look into the intake vacuum to ascertain if there's an aura leak that's upsetting the general air/fuel mixture. A few overlooked causes here might be a leaking EGR valve or perhaps a leaking energy brake booster.

Utilizing A SCAN TOOL

What's going to a scan tool let you know about misfire? Very little unless of course the automobile is outfitted with OBDII (1996 or more recent). Once the OBD II system picks up a misfire that surpasses "normal" limits, it fires up the Check Engine light and sets a P-code that matches the misfiring cylinder. The final number inside a P300 series code informs you which ones cylinder is misfiring. A code P304, for instance, states cylinder # 4 is misfiring. Should you also look for a P204 code (P200 series codes cover the injectors), you'd be aware of misfire was most likely triggered with a bad injector.

If you discover a P300 code, this means the misfire is random and it is getting around from cylinder to cylinder. The reason here would probably be something which upsets the engine's air/fuel mixture, like a major vacuum leak, leaking EGR valve or abnormally low fuel pressure (weak pump or faulty pressure regulator). There's really no quick fix for locating misfires. It requires some detective try to isolate the fault and see the main cause. So, next time you face a misfire, don't miss the objective.